Existing motion-picture film cameras designed for sync sound or mos (without sound) production utilize a single motor to drive the various mechanisms associated with film transportation and exposure. As shown in FIG. 1, the motor typically drives a main sprocket 104, which is coupled to a cam or geneva-type assembly to control the movement of "pull-down claws" 108. Once pulled down, or moved into place, a separate set of registration pins (not visible in the figure) are used to hold the film in place during exposure.
The single motor used in this type of camera may rotate at a continuous or variable speed, but it never stops. A variable speed is used only to increase or decrease the frame rate or the number of frames being exposed per second. A mirror or shutter rotates constantly between the rear lens element 112 and an opening 114 (known as the "gate"), where the film is exposed. Several gears are used to operate the mirror from the same motor used for film transportation. The mirror or shutter is commonly a disc shaped object 110 which is mirrored for 180 degrees and open for the additional 180 degrees. The mirrored portion is rotated in front of the gate as the film is being pulled or moved into place for exposure. After the registration pins have entered the film to hold it in place, the open 180 degrees rotates in front of the gate allowing light through the lens element to expose the film.
The speed of the single sprocket motor is commonly governed by a crystal for frame-rate accuracy. A few newer cameras use a microprocessor to govern speed. In any event, the single-motor concept requires complicated gearing to achieve the movement of the film, magazine feed/take-up displacement and mirror (shutter) rotation.
Some cameras have a variable shutter which allows an operator to adjust the amount of time the film is being exposed. This is accomplished by increasing or decreasing the degrees of the mirrored portion of the shutter; regardless, the shutter will still spin continuously and in relation to the speed setting of the single motor. Additionally, film supply (120) and take-up reels 122 are controlled from the single motor using a different set of gearing than used in conjunction with the shutter system. Control of "feed" and "take-up" is critical in existing cameras, as the torque required to pull or move the film through the gate is much greater than the film transport mechanism can handle. As a consequence, a "loop" (slack in the film path) such as 130 is used to lessen the pull requirement of the cam or geneva mechanism.
In addition to the type of mechanism just described, there does exist a class of cameras which incorporate a stepper-motor driven film transport. For example, in U.S. Pat. No. 4,360,254, to Nyman et al., entitled CAMERA HAVING STEPPER MOTOR DRIVEN FILM TRANSPORT/REGISTRATION SPROCKET, a single stepping motor is coupled to a sprocket used to advance the film bidirectionally in a `tight-loop` transport arrangement. A microprocessor is used to independently drive the film transport, magazine feed and take-up, shutter registration and mirror control.
Although the patent just discussed mentions that the camera may be used in movie animation as well as other applications, units of this kind are designed primarily for non-sync or non-action cinematography, and are therefore usually relegated to slide reproduction or animation. Accordingly, there exists an outstanding need for a direct-drive, servo-controlled film camera capable of being used for both on-location and non-sync or non-action cinematography.